Modern inorganic chemistry

86 ACIDS AND BASES: OXIDATION AND REDUCTION

and thus the conjugate acid H 3 O^ is not completely formed—such

acids are termed wea/c acids. (Again, more correctly, weak acids in

the solvent specified; HC1 is a weak acid in ethanol.) The strength of

an acid is measured by the position of equilibrium. For example, for

a weak acid in water

HA + H 2 O^H 3 O+ 4- A'

the equilibrium constant is given by

[H 30 +][A-]

[H 2 0] [HA]

However, in dilute solution [H 2 O] is virtually constant ([H 2 O] =

55.5 since 1 litre of water contains 1000/18 mol of H 2 O) and taking

this into the above expression for the equilibrium constant we obtain

a second constant

K ^[HsO+KA-]..-!

d~ [HA]

KA is known as the acid dissociation constant; it is a measure of the
strength of an acid in a particular solvent, which should be specified.
Values of JCa are small for weak acids and they range very widely
(Table 4.1). It is common practice to quote values as the negative
logarithm to the base ten, i.e. — Iog 10 Ka, since such numbers are
less cumbersome and positive when Ka < 1. The symbol for — Iog 10
is by convention^4 p', thus — Iog 10 Ka becomes pJCa. Table 4.1 shows
some typical pKa values.

Table 4.1

SOME pA^a VALUES FOR ACIDS IN WATER AT 298 K

Acid Ka mol 1 { pA'a

Ethanoic (acetic)

Methanoic (formic)

Hydrocyanic

Hydrofluoric

Hydrogen sulphide*

Benzenol (phenol)

1.75 x 1(T^5

1.77 x 10^^4

7.9 x 10 10

6.61 x 10"^4

10"

1.05 x 10 10

4.756

3.752

9.1

3.18

7.00

9.98

For the reaction H 2 S + H 2 O = HS - H_,Cr

For strong acids, Ka values are large and p/Ca values are negative,

for example pKa for hydrochloric acid is —7.